Google has decided to stop censoring its search results in China and may pull out of business in the country, after its systems came under a targeted cyberattack.

On Tuesday, the search giant's legal chief David Drummond posted a blog saying that, in mid-December, the company detected "a highly sophisticated and targeted attack on our corporate infrastructure originating from China that resulted in the theft of intellectual property from Google".

In the course of Google's investigation of the incident, it discovered that at least 20 other large companies — in the internet, technology, media and chemical industries — had been similarly targeted, Drummond said.

The investigation also uncovered evidence of an attempt to break into the Gmail accounts of Chinese human-rights activists, he said. The attempt failed, although the attackers did manage to access the account information of two individuals and obtained the subject lines — but not the bodies — of emails associated with those people's accounts.

Google also discovered that dozens of Gmail users in the US, Europe and China had had their accounts "routinely accessed by third parties", although Drummond noted that these compromises appeared to be separate from the attack on Google itself, and probably followed successful phishing scams.

Freedom of speech"We have taken the unusual step of sharing information about these attacks with a broad audience not just because of the security and human rights implications of what we have unearthed, but also because this information goes to the heart of a much bigger global debate about freedom of speech," Drummond wrote.

Google has operated in China since 2006, having agreed to meet the Chinese government's condition that it would block certain search results there. The company did this "in the belief that the benefits of increased access to information for people in China and a more open internet outweighed our discomfort in agreeing to censor some results", Drummond wrote.

However, the attacks and surveillance have led Google to reconsider whether it can operate in the country, he said.

"We have decided we are no longer willing to continue censoring our results on Google.cn, and so over the next few weeks we will be discussing with the Chinese government the basis on which we could operate an unfiltered search engine within the law, if at all," Drummond wrote. "We recognise that this may well mean having to shut down Google.cn, and potentially our offices in China."

Drummond also stressed that Google's US executives, not its employees in China, were responsible for the decision.

In a television interview with CNBC, Drummond said Google could "no longer in good conscience... continue to censor our search results [in China]". He stressed that Google was "not saying one way or the other whether the attacks were state sponsored or done with any approval of the state", but said it was clear that political dissidents and people interested in human rights in China were targeted.

No financial effectAsked how big an impact the closure of Google's Chinese activities might have on the company, Drummond said the business there was "quite small". He said opening the Chinese business in 2006 was "never really a financial move" for the company, and the revenues from that business were "truly immaterial", so the cutting off of that income would not have an effect on Google's financials.

Some cybersecurity experts, including those at Verisign iDefense, have drawn links between the attacks revealed on Tuesday and targeted attacks against 100 US companies made in July 2009. In the July attempts, technology companies were sent emails that included PDF files exploiting a vulnerability in Adobe Reader.

US secretary of state Hillary Clinton issued a statement on Tuesday saying the US government had been briefed by Google on its findings, which she said raise very serious concerns and questions.

"We look to the Chinese government for an explanation," Clinton said. "The ability to operate with confidence in cyberspace is critical in a modern society and economy. I will be giving an address next week on the centrality of internet freedom in the 21st century, and we will have further comment on this matter as the facts become clear."

In the UK, Labour MP Tom Watson is planning to table an early day motion in parliament, asking the House of Commons to praise Google on its decision to end its policy of censoring search results in China. The motion will also call on other technology companies to follow Google's lead, Watson wrote on his blog on Wednesday.